As severe as the effects of these high centrifugal forces are, HEAT
has a more detrimental effect. HEAVY LOADS and HIGH SPEEDS cause HEAT
GENERATION in aircraft tires to exceed that of all other tires.
To understand the magnitude
of heat generated in typical aircraft tires, several test tires
were fitted with temperature sensors, or thermistors, mounted at
the locations indicated. The actual temperature rise during a variety
of free-rolling taxi tests was monitored and recorded. The following
charts show the effect of taxi speed, inflation pressure, and taxi
distance on internal heat generation for typical main landing gear
tires.
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The vertical dotted line at
35 mph (30 knots) indicates the recommended maximum taxi speed.
On the above chart, the curves constantly slope upward with higher
taxi speeds. In other words, the faster an aircraft travels over
a given distance, the hotter the tires will become.
Many people would expect the
shoulder area to generate the most heat. In reality, the bead and
lower sidewall area are the hottest. There are two major reasons
for this:
1. All forces, in or acting
on a tire, ultimately terminate at the bead. This is an area of
high heat generation.
2. Rubber is a good insulator;
or said another way, it dissipates heat slowly. The bead area, being
the thickest part of the tire, retains the heat longer than any
other part of the tire.
This tire was designed to be
operated at 32% deflection, as the vertical dotted line indicates.
Left of this line designates overinflation, and to the right underinflation.
When the speed and the distance traveled are constant, the more
a tire is underinflated the hotter it becomes.
The rate of temperature rise
versus underinflation is the highest in the shoulder area due to
increased flexing. The bead area, however, still remains hottest.
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Even when an aircraft tire is
properly inflated and operated at moderate taxi speeds, the heat
generation will always exceed the heat dissipated. (This is indicated
by the ever increasing slope of the lines.) The farther the taxi
distance, the hotter the tires will be at the start of the take-off.
This chart shows the effect
of underinflation coupled with the high speed taxiing. A comparison
is made between a tire run at 32% deflection and one run at 40%
deflection. Not only is the slope of the 40% deflection curves much
steeper (due to higher rate of heat generation) than the 32% curve,
but the 40% deflection tire blew out in the lower sidewall after
traveling about 30,000 feet.
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The carcass or body of the tire
is made up of rubber coated layers of nylon fabric which extend
from bead to bead. This fabric, which is anchored to the bead bundles,
is a structural member of the tire to give it shape and strength.
As good as nylon is, it has
limitations. There is a reduction in strength when exposed to high
temperatures. Nylon melts at temperatures slightly above 400 degrees
F (200 degrees C).
Effect of Temperature on Rubber
Compounds
EFFECTS
Degrees F
Degrees C
APPEARANCE OF BLUE COLOR
210 - 230
100 - 110
RUBBER REVERTS
280 - 320
140 - 160
RUBBER BECOMES HARD & DRY
355 - 390
180 - 200
The physical properties of rubber
compounds are more susceptible to degradation by high temperature
than those of nylon. The compound is not significantly damaged when
temperatures are reached that cause bluish discoloration, but both
strength and adhesion are lost when the rubber reverts to the uncured
state. The temperatures shown in the above chart are related to
time. Brief exposure to these temperatures are not as damaging to
the tire as prolonged exposure.
On the previous charts it must
be remembered that only temperature rise was indicated. Heat is
cumulative. This chart shows the time required to cool the bead
area of a test tire with two fans blowing on it. This would equal
approximately a 30 mph breeze. The curve indicates that the temperature
in a hot tire drops 100 degrees F in the first hour and somewhat
less in subsequent hours. The cooling time of a tire mounted on
an aircraft would be slightly longer due to the effect of brake
temperature.
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High internal temperatures deteriorate
both compound and fabric, resulting in the following problems:
Tread & Carcass Separations
- Here we see separation in both shoulders. The wear pattern indicates
this tire was run underinflated.
Bead Face Damage - Up to now,
only heat generated internally has been discussed. This is an example
of damage due to external heat from the brakes.
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Courtesy
of The Goodyear Tire & Rubber Company
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